The steering flexible coupling is a potential safety critical weak point but you raise an interesting point about replacing stuff that many people dont consider:
The bath tub curve. This is a graph that describes the statistical failure rate of a item and unsurprisingly forms a bath shape in that there tends to be a lot of failures at the start of its usage. Then as time passes the failure rate reduces and stays lower until the end of its life when the number of failures increases again.
All this means that there is a certain risk when you replace any component that you are removing something that is in the lowest stage of the curve and returning to the start with a much higher risk of failure. This higher risk includes, as in Marks case, incorrect installation or it can be a defect in the new component itself from design, quality or manufacturing issues.
Its something thats taken very seriously in industry and we should consider because quite often the risk of an item failing can be, paradoxically, greater if we replace it!